A model study of the seasonal circulation in the Gulf of Maine

The Princeton Ocean Model is used to study the circulation in the Gulf of M aine and its seasonal transition in response to wind, surface heat flux, ri ver discharge, and the M-2 tide. The model has an orthogonal-curvature line ar grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical. It is initialized and forced at th e open boundary with model results from the East Coast Forecast System. The first experiment is forced by monthly climatological wind and heat flux fr om the Comprehensive Ocean Atmosphere Data Set; discharges from the Saint J ohn, Penobscot, Kennebec, and Merrimack Rivers are added in the second expe riment; the semidiurnal lunar tide (M-2) is included as part of the open bo undary forcing in the third experiment. It is found that the surface heat flux plays an important role in regulatin g the annual cycle of the circulation in the Gulf of Maine. The spinup of t he cyclonic circulation between April and June is likely caused by the diff erential heating between the interior gulf and the exterior shelf/slope reg ion. From June to December the cyclonic circulation continues to strengthen , but gradually shrinks in size. When winter cooling erodes the stratificat ion, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the ene rgy is consumed by bottom friction. While inclusion of river discharge chan ges details of the circulation pattern, the annual evolution of the circula tion is largely unaffected. On the other hand, inclusion of the tide result s in not only the anticyclonic circulation on Georges Bank but also modific ations to the seasonal circulation.